1. Field of the Invention
This invention relates generally to processes for the treatment mercaptans. More specifically this invention relates to processes for the removal of mercaptans from normally gaseous hydrocarbons streams.
2. Description of the Prior Art
The sweetening of sour hydrocarbons streams by the conversion or removal of mercaptan sulfur is well known. Mercaptans present in such feedstreams are converted by the sweetening process to disulfide compounds. In the sweetening process the mercaptan containing hydrocarbon contacts a mercaptan oxidation catalyst carried by an alkaline solution in the presence of an oxygen supply stream. Typically in the performance of the sweetening process the disulfides remain in the hydrocarbon stream and are, therefore, not removed but converted to an acceptable form.
A wide variety of processes are known for the sweetening of distillates. U.S. Pat. No. 4,490,246 and the references cited therein set forth a number of flow schemes for the sweetening process. A number of different separation arrangements can be used to recover the treated distillate and the catalyst containing alkaline stream. The '246 patent seeks to reduce the separation of dissolved disulfide gases from a liquid product and teaches the use of a settler and a low pressure separator to remove a gaseous phase of disulfides from the product effluent of the sweetening process. As demonstrated by U.S. Pat. No. 2,988,500 a single settler can be used to withdraw excess gases overhead, a product stream from an intermediate section of the settler and a bottoms stream of an alkaline catalyst solution.
Extraction processes are typically used when treating light hydrocarbons and gas streams for mercaptan removal. In the extraction process the feed first contacts a caustic solution in an extraction column. The caustic solution contains a mercaptan oxidation catalyst. Feed depleted in mercaptans passes overhead from the extraction column and the mercaptan containing caustic passes countercurrently from the bottom of the column. The mercaptan rich caustic receives an injection of air and catalyst as it passes from the extraction column to an oxidizer for the conversion of mercaptans to disulfides. A disufide settler receives the disulfide rich caustic from the oxidizer. The disulfide settler vents excess air and decants disulfides from the caustic before the caustic is returned to the extractor.
The above described extraction flow scheme can be used to remove mercaptans from fuel gas streams in refineries. In such arrangements the feed is contacted under gaseous conditions. However, such schemes have been found to be unsatisfactory in reducing sulfur concentrations to very low levels when the feed streams have a continuous or intermittent oxygen concentration. The presence of oxygen in the feed leads to oxidation of the mercaptans to disulfides in the extractor. These disulfides are stripped from the caustic by the volatile fuel gas and raise the total sulfur concentration of the fuel gas product to unacceptable levels for environmental standards.
Other methods are known to reduce the sulfur concentration of mercaptan containing gas streams. U.S. Pat. No. 4,808,341 issued Feb. 28, 1989 discloses a process for the separation of gases from mercaptans, the process uses a lean oil to absorb mercaptans in a first contacting zone and regenerates the absorption oil by contacting the mercaptan rich oil with an aqueous oxidizing agent to produce a sulfuric acid solution and a hydrocarbon absorption oil.